It is desirable to encode television signals to provide secure transmission in applications such as cable TV, satellite tranmission and subscription broadcast television. Known coding arrangements may be considered, generally speaking, to be of either the amplitude alteration type or of the time sequence alteration type. Amplitude encoders include, illustratively, those which alter the vertical or horizontal synchronizing pulse amplitude or which modulate the video signal with an encoding waveform (e.g., a sine wave) and have an advantage in that the decoder may be relatively simple and inexpensive. A disadvantage of amplitude encoding is that, generally speaking, the codes are rather easily recognized and deciphered, thus creating a potential "black market" for unauthorized decoders. From a technical standpoint, the amplitude distortion imparted to the video signal may not be completely removed by the decoder and critical decoder adjustments may be required to reduce the residual distortion of the decoded signal to acceptable levels. Other problem areas associated with scramblers of the amplitude alteration type include signal-to-noise ratio degradation and loss of dynamic range.
Encoders of the time sequence alteration type reorder the video signal sequence so as to create a non-standard signal sequence which may be changed at random so as to provide a very high level of security. Examples of such encoders are those which reverse the sequence of picture elements within a line, those which transpose or interchange lines within a field and those which transpose or rearrange segments of each line of the video signal as described in U.S. Pat. No. 4,070,693 to Shutterly. In addition to the advantage of high security, such arrangements are generally characterized by very low levels of residual amplitude distortion but heretofore have required relatively complex (and expensive) encoders and decoders owing, mainly, to the requirement for storing the video signal during encoding and decoding.